Why does the shuttle throttle on ascent?

"Henry Spencer" wrote in message
...
In article ,
MichaelJP wrote:
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.

Is it also the case that the zero-lift trajectory you describe is very
similar to the optimum flight path for orbital insertion? Or is a lot more
fuel used because of it?

Yes and no. :-)

If memory serves, the ascent trajectory is pretty close to what a wingless
rocket with similar mass and propulsion characteristics would fly. Flying
even slightly sideways at supersonic speeds is very hard on lightweight
structures; even jet fighters, built for violent maneuvering, can handle
only a very little bit of this. Rockets normally take considerable pains
to fly pretty much(*) straight "into the wind" until clear of most of the
atmosphere. The shuttle trajectory isn't *exactly* what a wingless rocket
would use, because the trajectory that minimizes loads on the orbiter
wings isn't exactly the trajectory that would minimize structural loads in
general -- the wings have priority. But the penalty for this is small.

(* There are minor exceptions, in which lift can be of some use after the
air thins out, plus some complications for air-launched rockets like
Pegasus. But this is still basically correct. )

*However*, there is a more general caveat: even the wingless-rocket
trajectory actually isn't optimal. For one thing, an optimal ascent would
tip over toward the horizontal much more quickly. On Earth, the early
ascent has to be close to vertical, to get the rocket up out of the
atmosphere before the speed builds up too much. For another thing, even
disregarding that, the straight-into-the-wind trajectory isn't exactly
optimal, although it's not too far off.

The only rocket ascent that was ever able to use a truly optimized
trajectory was the Apollo LM ascent stage's departure from the Moon. On
Earth, you inevitably pay some price for the necessities of getting clear
of the atmosphere quickly and pointing straight into the wind while you
do. It's not huge, but it's significant. This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)
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Thanks for the extra detail!

MichaelJP wrote:
This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)

Thanks for the extra detail!



There's another advantage if you're using cryogenic propellants. The
propellants can be kept in insulated tankage within the carrier until
altitude is reached and the transferred into the LV. Since the
temperature is well subzero at altitude, there isn't water vapor around
to form ice on the tankage, so the weight and complexity of insulation
can be done away with.
Assuming you are using a Shuttle-style jettisonable ET, that a built-in
performance boost, as well as a cost savings on the ETs themselves.
Although a completely rreusable LV will have a TPS to take reentry
heating, and therefore will already have exterior insulation, the drop
tank solution makes for far easier design as far as vehicle weight goes.

Pat

In article ,
Pat Flannery wrote:
This is one of the two big technical advantages of air launch...

There's another advantage if you're using cryogenic propellants. The
propellants can be kept in insulated tankage within the carrier until
altitude is reached and the transferred into the LV. Since the
temperature is well subzero at altitude, there isn't water vapor around
to form ice on the tankage, so the weight and complexity of insulation
can be done away with.

You don't really need insulation against ice anyway, unless you've been
stupid enough to put something fragile downstream of the tank surfaces.
Just let it fall off after engine ignition, as the Saturn V did.

The big reason why you might need tank insulation is if the tank holds
LH2, in which case you need to insulate to prevent liquid air from
condensing... and that'll happen even at subzero temperatures, so you
can't get away with leaving it off.

Although a completely rreusable LV will have a TPS to take reentry
heating, and therefore will already have exterior insulation, the drop
tank solution makes for far easier design as far as vehicle weight goes.

The gain is actually rather questionable, after you consider reentry --
the drop tank leaves behind a heavy, dense vehicle that makes a severe
reentry. At reentry time, it's *good* if lots of the volume inside the
TPS is empty tanks. The drop tank does make for far easier design if you
can "throw the TPS problem over the fence" to the materials team...
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Henry Spencer wrote:

The big reason why you might need tank insulation is if the tank holds
LH2, in which case you need to insulate to prevent liquid air from
condensing... and that'll happen even at subzero temperatures, so you
can't get away with leaving it off.


In the case of a carrier aircraft, the airstream should carry away any
liquid air on the tank.


Although a completely rreusable LV will have a TPS to take reentry
heating, and therefore will already have exterior insulation, the drop
tank solution makes for far easier design as far as vehicle weight goes.


The gain is actually rather questionable, after you consider reentry --
the drop tank leaves behind a heavy, dense vehicle that makes a severe
reentry. At reentry time, it's *good* if lots of the volume inside the
TPS is empty tanks. The drop tank does make for far easier design if you
can "throw the TPS problem over the fence" to the materials team...

I'm really surprised that the small air-launched orbiter with giant drop
tank concept didn't get anywhere- both we and the Russians thought the
idea had enough merit to do designs of the concept:
http://www.buran.ru/htm/busfact.htm#maks-op
http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld053.htm

Pat

In article ,
Pat Flannery wrote:
The big reason why you might need tank insulation is if the tank holds
LH2, in which case you need to insulate to prevent liquid air from
condensing...

In the case of a carrier aircraft, the airstream should carry away any
liquid air on the tank.

With any luck, assuming it doesn't go somewhere it shouldn't... but the
condensation will still produce a massive heat flux into the LH2 tank, and
it doesn't take much to boil LH2.
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Pat Flannery wrote:

http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld053.htm

A 747 with an SSME in the tail...

[Note to r.a.p. - that's a Space Shuttle Main Engine]

Mary Pegg wrote:
A 747 with an SSME in the tail...
The _New_ Boeing SST proposal! ;-)
It makes sense though... since you're storing LOX and LH2 on the 747
anyway for pumping into the ET at altitude, you might get some use out
of them to up its maximum release altitude at the same time.

Pat

In article ,
Pat Flannery wrote:
A 747 with an SSME in the tail...

...It makes sense though... since you're storing LOX and LH2 on the 747
anyway for pumping into the ET at altitude, you might get some use out
of them to up its maximum release altitude at the same time.

If memory serves, Boeing eventually concluded that this wasn't a good way
to do it -- too much structural strengthening needed? They looked instead
at injecting fuel into the bypass ducts of the turbofans (!), and
concluded that they could get a very large thrust increase that way, and
that it wouldn't hurt the engines if you kept the duration short.
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spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article ,
Pat Flannery wrote:
I'm really surprised that the small air-launched orbiter with giant drop
tank concept didn't get anywhere- both we and the Russians thought the
idea had enough merit to do designs of the concept...

Or without drop tank. Grumman's proposal to the SDIO SSTO competition
(won by McDD with DC-X) was a no-drop-tank orbiter, air launched from a
747, that used existing engines and quite ordinary structures.
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